Green roofs are becoming more and more mainstream. But what are they for and how do they work? GRO explains.
Green roofing advocates cite a long list of benefits from green roofing, but can they really be true? Is it possible for a few plants and a layer of growing medium to provide all of those services – and for free? Let’s take a look at the advantages of green roofing and discover how each one comes about.
Benefits Of Green Roofing
- Stunning good looks
- Enhances a building’s thermal performance
- Stormwater retention
- Biodiversity net gain
- Cools buildings
- helps reduce the urban heat island effect
- Increases the life of the waterproofing
Stunning good looks
A living roof needn’t be just sedums – although they are beautiful – green roof design should be all about year round interest for as many species as possible
This is one green roof benefit that is self explanatory. But did you also know that there are also huge benefits in looking out onto an expanse of green rather than boring old waterproofing. We’re talking wellbeing, productivity and of course PR. Lots to discuss, but if it’s OK with you, I’ll leave that for another blog and move on to thermal performance.
How Does a Green Roof Improve the Thermal Performance Of A Building?
“How?” I hear you ask “Does spreading dirt on the top of a building help to keep it cooler in summer and warmer in winter?”
Well, let’s think about some other situations where extra layers help to regulate thermal performance.
Caves – there’s a reason our ancestors chose to live in caves rather than in trees. Because caves are generally underground – in other words they have layers of rock and dirt on top of them – they stay at a pretty consistent temperature all year round. Which means that in summer time, they feel cooler than the outside. And in winter time they feel moderately warmer. Is that not what we want for our buildings?
Horses and babies. Weird analogies I know, but bear with me.
Have you ever noticed at the end of a stint on the racecourse or the show jumping ring, a groom will appear from nowhere and throw a blanket full of holes over the creature’s back before walking them round and round for a while. That blanket is called a ‘sweat rug’ (yuck) and it’s designed to bring the animal’s body temperature down slowly in order reduce the risk of illness of muscle stiffness. (Think London Marathon runner + space blanket). What happens with a sweat rug, is air gets trapped in those holes, absorbs excess heat energy from the animal’s body and slowly disperses it into the atmosphere.
A baby’s lacy shawl works in the same way. It captures air and creates a breathable barrier between the little body and the environment. Babies are notoriously bad at regulating their own body temperature and a lacy shawl or blanket is a traditional way of ensuring they are neither too hot nor too cold. See – Grandma really did know best!
How do baby’s shawls and sweaty horses relate to green roofing? Well, both the substrate layer of a living roof and the vegetated layer contain masses of air holes which act as a regulatory mechanism against temperature changes. The deeper the green roof build up, the more air pockets it has, and the more effective it is at protecting a building against internal and external temperature changes.
Giving a building a living green roof is a superb way of improving its thermal performance at any time of year
Stormwater Retention
Here in the UK there’s a lot of work needed to manage stormwater runoff more effectively. Our drainage systems are frequently overwhelmed during heavy rain events which can lead to flooding. Not only that, it costs a fortune to treat the runoff from roads and pavements before it can be returned to our waterways. So what would Mother Nature do?
Actually, Mother Nature has got it sorted. It’s only because we’ve disrupted her perfect system by burying soil beneath buildings and infrastructure that we have any problems at all. Nature’s way is for rain to be absorbed by soil, cleaned by plants and soil microbes, filtered through rocks into aquifers and then slowly released into streams and rivers. As it travels through its journey its temperature is matched to the earth so that it doesn’t shock aquatic life forms when it finally reaches a river.
Guess what. Green roofs are brilliant at emulating that first part of Nature’s water management process. Rainwater is retained by the green roof buildup. From there it is either used by the plants to support life, slowly evaporated into the atmosphere (cooling the air in the process), or stored by the roof’s drainage system.
Any excess trickles slowly off the roof, so as not to overwhelm drainage systems. In an ideal world it will be directed to other SuDS systems, rain gardens or reservoirs where it can continue to benefit the world at large without the involvement of chemical treatment systems.
Green Roofs and Biodiversity Net Gain
Biodiversity loss statistics for the UK are horrifying. . One report finds that there has been an almost 70% decline in wildlife populations since the 1970’s. Since many elements in our food chain are directly or indirectly affected by insects, birds, fish, reptiles, amphibians and mammals. It’s time to stand up and take notice. But how can green roofs help?
Humans need homes, shops, meeting places and highways in order to survive. And so do creatures. Well, maybe not shops, but they certainly need a source of food. The trouble is, by building homes etc for ourselves, we’re destroying the habitat of the creatures that we depend upon.
Instead of destroying wildlife habitat – why not just lift it up out of our way? That’s exactly what a green roof does. OK, maybe you won’t get badgers, wild boar or beavers living on your green roof, but it can certainly give provide shelter, food and highways for an immense variety of insects and birds.
Why do wildlife highways matter?
Because breeding birds or insects need to meet others from their own kind from different communities. Inbreeding is not good for any species, especially in a rapidly changing climate where survival of the fittest is more relevant than ever.
Don’t forget the value of green roofing to plant species. I know it’s a figure that’s continually being bandied about, but we’ve lost 98% of our wildflower meadows since the 1940’s. Wildflowers are not just pretty, they’re not just food for insects. Many of them are the source of the medical knowledge we all benefit from today.
Over time, ancient ‘old wive’s’ cures have morphed into modern drug treatments and I for one, am convinced that there’s still more to learn. But how can we learn if we’ve lost those plant species? We can’t. So let’s use green roofing to preserve the gene stock for future generations to make use of.
Cooling The Urban Heat Island Effect
The Urban Heat Island Effect results in towns and cities feeling warmer than rural areas
It’s an actual fact that towns and cities are warmer than the surrounding countryside. That’s nice in winter, very uncomfortable (and unhealthy) in summer. Why is that do you suppose it happens? Is it that having many buildings close together shelters us from cruel winds? That could be part of it. But in fact, scientists partially attribute the Urban Heat Island Effect to albedo. Al who? Albedo is a term used to describe how the sun’s energy is bounced off surfaces back into the atmosphere, thus warming the air two or more times.
Dark coloured surfaces – for example roads and standard rooftops, tend to absorb energy whilst light coloured surfaces – eg arctic snow, bounce the sun’s energy back into the atmosphere. The more energy is absorbed, the warmer an area will be. Some people believe that painting walls and roofs white is a good way to deflect heat. That may be so in summer, but is it really what’s needed in winter?
The plants on the green roof also ‘sweat’ themselves. Called transpiration, this constant loss of water also helps to cool the air above a green roof, making it much cooler during hot spells.
Now let’s bring transpiration into the equation. Transpiration is nature’s way of stopping plants from being cooked in the heat of the sun. Tiny pores in the surface of the leaves release water vapour into the atmosphere which cools both the plant and the air around it. In addition, those water molecules floating around in the air will reflect the sun’s energy back into the higher atmosphere. That’s why it always feels cooler beside a body of water than it does in the middle of a tarmac car park.
How does a green roof work against the urban heat island effect?
Firstly it absorbs some of the sun’s energy so that it doesn’t get pinged around from building to building. And secondly all of those wonderful living plants are contributing to the transpiration that will also deflect energy into the higher atmosphere.
The more plants and the more green areas we can introduce into our cities, the better we will be able to mitigate against the urban heat island effect. A green roof doesn’t take up any space on the ground and therefore will not impact on our infrastructure. It’s a win-win situation.
Increasing the Longevity Of Waterproofing
Have you ever had to repair the roof of your home? Expensive isn’t it? And however much does it cost the planet when spent materials are sent to landfill and new waterproofing is manufactured and delivered?
Why does waterproofing deteriorate over time? It’s largely as a result of enormous temperature changes causing the material to expand and contract more often than it really wants to, as well as being constantly pounded with UV rays from the sun. During the summer months exposed membranes could reach 50-60oC, and in winter, temperatures below freezing. A green roof acts as a buffer and reduces the severity of maximum and minimum temperatures and shelters it from direct UV rays, thereby extending the life of the membrane.
You’ll find more fascinating insights into how green roofs work in the GRO Green Roof Code. Not only does the document set out best practice for designing, specifying and installing living roofs. It also tells you WHY green roofs are so important.